1. Field of the Invention
The present invention relates to an R—Fe—B-type rare earth magnet, an alloy powder for such a rare earth magnet, a method of making the powder, and a method for producing the magnet.
2. Description of the Related Art:
A rare earth sintered magnet is produced by pulverizing a material alloy for the rare earth magnet to obtain an alloy powder, compacting the alloy powder, sintering the compact and then subjecting the sinter to an aging treatment. The rare earth sintered magnets extensively used today for various applications are roughly classifiable into the two types, namely, samarium-cobalt-type magnets and rare earth-iron-boron-type magnets. Among other things, the rare earth-iron-boron-type magnets (which will be referred to herein as “R—Fe—B-type magnets”, where R is one of the rare earth elements including Y, Fe is iron and B is boron) recently have been extensively applied to various types of electronic apparatuses. This is because an R—Fe—B-type magnet can exhibit a higher magnetic energy product than any other type of permanent magnet and yet is relatively inexpensive. It should be noted that a transition metal element such as Co may be substituted for a portion of Fe in the R—Fe—B-type magnet, and carbon may be substituted for a portion of boron.
A powder of a material alloy for an R—Fe—B-type rare earth magnet is sometimes prepared by a method including first and second pulverization processes. That is to say, the material alloy is coarsely pulverized in the first pulverization process and then the coarsely pulverized alloy is finely pulverized in the second pulverization process. More specifically, the material alloy is embrittled in the first pulverization process by utilizing a hydrogen occlusion phenomenon so as to be coarsely pulverized to sizes of several hundreds of micrometers or less. Thereafter, in the second pulverization process, the coarsely pulverized alloy (or coarsely pulverized powder) is finely pulverized to a mean particle size that is several micrometers using a jet mill machine or other suitable apparatus.
Methods for preparing the material alloy itself may also be generally classifiable into the two types: ingot casting and rapid cooling processes. Specifically, in an ingot casting process, a melt of the material alloy is poured into a casting mold and cooled in the casting mold relatively slowly. Typical examples of the rapid cooling processes include a strip casting process and a centrifugal casting process. In the rapid cooling process, a melt of the material alloy is brought into contact with, and rapidly cooled by, a single roller, twin rollers, a rotating chill disk, a rotating cylindrical chill mold or other similar device, thereby making a solidified alloy that is thinner than an ingot cast alloy.
In a rapid cooling process as described above, a melt of a material alloy is normally cooled at a rate of 102° C./sec to 2×104° C./sec. A rapidly solidified alloy prepared by the rapid cooling process usually has a thickness of 0.03 mm to 10 mm. The melt starts to solidify upward at the lower surface thereof that is in contact with a chill roller (which surface will be referred to herein as a “roller contact surface”). From the roller contact surface, crystals in the shape of pillars (columns) or needles grow upward in the thickness direction. As a result, the rapidly solidified alloy has a microcrystalline structure including an R2T14B crystalline phase and an R-rich phase. Fine crystal grains of the R2T14B phase have a minor-axis size of 0.1 μm to 100 μm and a major-axis size of 5 μm to 500 μm. The “R-rich phase” as used herein means a non-magnetic phase in which a rare earth element R is present at a relatively high percentage. The R-rich phase is dispersed around the grain boundaries of the R2T14B phase. The thickness of the R-rich phase (corresponding to the width of the grain boundaries) is 10 μm or less.
Compared to an ingot cast alloy, i.e., an alloy prepared by the known ingot casting (or mold casting) process, the rapidly solidified alloy has been cooled in a relatively short time. Thus, the rapidly solidified alloy has a finer structure with smaller crystal grain sizes. Also, in the rapidly solidified alloy, crystal grains are finely dispersed, the grain boundaries thereof have a wider area and the R-rich phase is distributed thinly over the grain boundaries. Accordingly, the rapidly solidified alloy is also advantageous in the dispersion of the R-rich phase.
After a rapidly solidified alloy such as that described above has been pulverized by the above-described techniques, the resultant powder is compacted using presses, thereby obtaining a powder compact. Also, by sintering this powder compact, an R—Fe—B-type rare earth magnet can be obtained.
In the prior art, a block-shaped sintered magnet, which is greater in size than a size of the final magnet product, is formed and then cut and/or processed to obtain a magnet having a desired shape and size.
Recently, however, a sintered magnet having a non-ordinary complex shape (e.g., arced shape) is in high demand. In response to this demand, even an as-pressed powder compact should sometimes have a shape that is close to that of a final magnet product. To make a compact having such a complex shape, a pressure to be applied to the powder being pressed and compacted (which pressure will be herein referred to as a “compaction pressure”) should be reduced compared to the known process. In producing an anisotropic magnet, the compaction pressure is low to increase the degree of magnetic alignment of the powder particles.
If the compaction pressure is reduced, however, the resultant compact density is reduced, and eventually its strength is decreased. As a result, the compact easily cracks or chips when the as-pressed compact is unloaded from the die cavity of the press or in any of the various succeeding process steps. In particular, an alloy powder for an R—Fe—B-type rare earth magnet often has an angular shape and has a compactibility that is inferior to those of other magnet material powders. Also, if the material alloy has a fine structure as in a strip cast alloy, then the powder obtained by pulverizing such an alloy should have a sharp particle size distribution. Accordingly, the springback (i.e., the elastic recovery of a compact that is observed when the compaction pressure applied to the powder is released) is remarkably observed in such a compact. As a result, the compact also likely cracks or chips. When the compact cracks or chips in this manner, the production yield drops, thus increasing the production costs disadvantageously. What is worse, valuable material resources cannot be utilized effectively enough. Problems like these are particularly noticeable if, while a material alloy for an R—Fe—B-type rare earth magnet is finely pulverized with a jet mill, for example, powder particles of relatively large sizes are screened out using a classifying rotor to increase the coercivity of the resultant magnet.